1. Field of the Invention
The present invention is directed to deposited thin films of semiconductors and dielectrics. The present invention further relates to the use of these thin films in separation, release, and sacrificial layer applications. Separation layer and release layer applications of these thin films include functions for separating materials and structures in fabrication processing for fields such as microelectronics, displays, solar cells, sensors, detectors, opto-electronics, biotechnology, and micro-electro-mechanical (MEMs) devices and systems. Sacrificial layer applications of these thin films include sacrificial film functions for the creation of void regions for uses such as channels, tubes, “air-gaps”, and cavities for microfluidics, separation/sorting structures, fuel cells, dielectrics, acoustic structures and optical structures.
2. Description of the Related Art
Separation layer approaches are used to physically separate systems of materials into at least two distinct systems. The approach is based on the use of some separation material positioned between other materials such that the separation material may be etched, mechanically abraded, or dissolved away leaving at least two, physically separated, distinct groupings of materials. Release layers are similarly used; however, in release layer applications the two materials systems are not totally separated. The separation or release layer material is often a polymer (e.g., photoresist), silicon dioxide, or polycrystalline silicon (Sang-Gook Kim and Kyu-Ho Hwang, Information Display, 15, 30 (1999). Recently, electrochemically etched porous silicon has been used as a separation layer for producing silicon on insulator (SOI) wafers for microelectronics applications (T. Yonehara and K. Sakaguchi, Abstract # 438, The Electrochemical Society, Fall Meeting, October, 2000, Phoenix, Ariz.).
Sacrificial layer approaches are used to create micro-scale and nano-scale void or cavity regions. Such voids or cavities are an enclosed empty space, which may be subsequently filled. Void creation is accomplished by removing a sacrificial material thereby leaving an empty region enclosed by an envelope of one or more materials. The size and shape of the void or cavity can be designed for the specific application. The shape may be varied and serve a variety of functions such as a channel, tube, “air-gap”, or cavity. Commonly used sacrificial layer materials include polymers, silicon dioxide, and poly-crystalline silicon (M. B. Stern, M. W. Geis and J. E. Curtin, J. Vac. Sci. Technol. Vol. B15(6), pp. 2887 (1997) and S. W. Turner and H. G. Craighead, Proc. SPIE Vol.3258, pp. 114 (1998)).
Deposited or thermally grown silicon dioxide and deposited poly silicon are probably the most commonly used sacrificial materials (P. J. French, J. Micromech. Microeng. Vol. 6, pp. 197 (1996) and S. Sugiyama, O. Tabata, K. Shimaoka and R. Ashahi, IEDM Tech. Dig. pp. 127 (1994)) and their etch rate can be relatively very high when they are deposited on an open region. When used as the sacrificial layer in the creation of a void structure, such as, a channel, tube, cavity, or “airgap”, these materials will, of course, be covered by a cap layer, which forms the “roof” of what will become the void region. The void or cavity region is then formed by etching away the sacrificial layer material through a window or through holes in or beside the cap layer. This window provides etchant access and reaction product exit. Consequently, the etch rate can become very low because the etch rate depends on the transport processes of the etchant solution, of the reaction products, or of both rather than just chemical reaction rate. That is, the removal of the sacrificial layer depends on the access of the etchant to and the removal of the reaction product from the sacrificial layer material as well as chemical etch rate. Consequently, materials that may have fast etch rates when deposited in open regions often have considerably slower etch rates when used as sacrificial layers.
Conventional porous silicon, produced by electrochemical etching of silicon, has also been tried for sacrificial layer application. (T. E. Bell, P. T. J. Gennissen, D. DeMunter and M Kuhl, J. Micromech. Microeng. Vol. 6, pp. 361(1996) and P. Steiner, A. Richter and W. Lang, J. Micromech. Microeng. Vol. 3, pp. 32 (1993)). However, the use of the material is hampered due to the following: its lack of uniformity and controllability, the necessity of having electrical conduction paths for the electrochemical etching required to create the material, the fact that it must be formed on a conductor, and the residual impurities left in the material after its electrochemical etching.
In general, there are a number of ways to produce void structures in addition to using sacrificial layers. All of the approaches, including the use of sacrificial layers, may be classified into two basic methods. The first, bulk micromachining, the substrate-to-substrate or wafer-to-wafer bonding technique, creates features in surfaces using standard processes such as etching, milling, embossing, stamping, etc. and then bonds a substrate and capping wafer or substrate, thereby creating nano- or microchannel features. In principle, this bonding approach is a relatively simple process. However, it requires anodic or direct (fusion) bonding and has the critical disadvantage of needing alignment of top and bottom. This makes the fabricating of small channel dimensions difficult because of misalignment of the two substrates and micro void formation at the bonding interface during the bonding process. The second technique, surface micromachining, is the method based on the use of sacrificial layers. Since it is based on sacrificial layer use, it can produce channel dimensions down to a few nanometers although to date the sacrificial layer removal step has been a relatively complex process (M. J. de Boer, W. Tjerkstra et al., J. of Microelectrochemical systems, Vol. 9, No. 1, pp. 94, March 2000). Between these techniques, surface micromachining is considered to be the most reliable method to fabricate fine structures and it certainly is the most reliable method for applications, such as optical resonate cavities, which require strict structure dimensions.
The present invention is based on using separation and release layer material and sacrificial material approaches in device fabrication. Specifically, it is based on using a new material for separation, release and sacrificial applications and a new, simple processing flow for separation layer, release layer and sacrificial layer implementation. The new materials of the present invention are deposited large material surface to material volume ratio thin films. The large surface to volume ratio insures a large empty region between material (i.e., material volume) regions allowing easy access by etching chemicals and easy reaction product removal. It insures the materials essentially wet very uniformly leading to very uniform attack and removal. In addition, the large surface area assures efficient chemical attack of the material when exposed to removing chemicals. The large surface to volume structure also leads to a mechanically weakened material for which mechanical agitation can be helpful in removal or for which gases trapped in this material may be used to enhance removal. With the materials of the present invention, procedures for separation layer applications and the removal procedure for sacrificial layer applications are more reliable than the other release and removal processes, are faster than other release and sacrificial layer approaches, and allow close process control. In addition, these novel materials are deposited and, therefore, can be used with a variety of substrates, including, but not limited to, plastics, glasses and metal foils.